I just used Atlanta Bread Company's bandwidth to upgrade to QuickTime 7. The previews in the "HD Theater" are simply amazing - no pixellation on resizing, and you can resize them on the fly.
My piddly little 1024x768 iBook screen can't even display the complete low res picture - it's too wide, but since it can be scaled anywhere inbetween without any loss in (easily perceived) detail, it's not a big issue. The copy on the Apple site says that to really pump the HD bits, you need a G5. Maybe for the 1080i version, but the regular "Batman Begins" trailer is gorgeous on my 1 Ghz G4.